A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to store compressed air and to use the stored, compressed air to supplement the air charge from the turbocharger compressor. For example, Pursifull et al. describe a system in US 2011/0132335 wherein compressed air is stored in a boost reservoir and is dispensed into the intake manifold when insufficient compressed air is available from the turbocharger compressor. In particular, the boost reservoir is charged with fresh intake air and/or effluent from one or more unfueled cylinders. By dispensing extra compressed air from the boost reservoir to the intake manifold, torque corresponding to the dispensed air can be provided to meet the torque demand while the turbine spins up.
However, the inventors herein have identified potential issues with such a system. As one example, if the boost reservoir has a small volume, the boost air may be initially able to supply sufficient air to provide increased desired torque, but after the supply is depleted, such as at higher engine speeds, the turbine may still not be spun up, and thus torque may drop following the initial increase. Such performance may be worse than no compensation at all. Further still, the pressure of the air dispensed by the boost reservoir may not be sufficiently high to overcome the boost pressure, or the charge in the reservoir may be primarily exhaust gas and thus provide little excess oxygen for combustion to compensate the turbo lag.
Thus, at least some of the above issues may be addressed by a method for a turbocharged engine. In one embodiment, the method comprises, in response to a tip-in, reducing turbo lag by discharging pressurized charge from a boost reservoir to an exhaust manifold. In this way, pressurized charge is discharged into the exhaust manifold to rapidly increase exhaust pressure.
For example, during previous engine operation prior to the tip-in, the boost reservoir may already be filled with primarily combusted exhaust gas from the exhaust manifold. In response to the tip-in, the pressurized charge including the combusted exhaust gas may be discharged into the exhaust manifold. As a result, an exhaust pressure at the turbine may be increased to expedite turbine spool-up. By providing the charge to the exhaust manifold, it can be more easily spread over a longer duration as the turbine consumes the gas at a lower rate than the engine induction. In this way, the increased exhaust pressure can assist in compensating for turbo-lag, and maintain a continuously increasing engine output while responding to the tip-in.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.